Disable preview for unsupported wiki formats

Summary

The preview in the wiki editor always assumes Markdown and doesn't work correctly when another format is selected.

Improvements

• Disable previewing when a format other than Markdown is selected.
• This needs to react to the Format dropdown in the form.
• If the current format is Markdown and the Preview tab is selected, switching the format should switch back to the Write tab.

We'll implement proper preview support for each format in follow-up issues.

Involved components

• app/views/shared/wikis/_form.html.haml
• app/assets/javascripts/pages/shared/wikis/wikis.js